An acoustic transducer is an electronic device used to emit and receive sound waves. Ultrasonic transducers are acoustic transducers that operate at frequencies above 20 KHz, and more typically, in the 1–20 MHz range. Ultrasonic transducers are used in medical imaging, non-destructive evaluation, and other applications. The most common forms of ultrasonic transducers are piezoelectric transducers. Recently, a different type of ultrasonic transducer, the capacitive microfabricated ultrasonic transducer, has been described and fabricated. Such transducers are described by Haller et al. in U.S. Pat. No. 5,619,476 entitled “Electrostatic Ultrasonic Transducer,” issued Apr. 9, 1997. The Haller patent describes transducers capable of functioning in a gaseous environment, such as air-coupled transducers. Ladabaum et al., in U.S. Pat. No. 5,894,452 entitled, “Microfabricated Ultrasonic Immersion Transducer,” issued Apr. 13, 1999, describe an immersion transducer (i.e., a transducer capable of operating in contact with a liquid medium), and, in U.S. Pat. No. 5,982,709 entitled, “Acoustic Transducer and Method of Microfabrication,” issued Nov. 9, 1999, describe improved structures and methods of microfabricating immersion transducers. In U.S. Pat. No. 6,271,620 entitled, “Acoustic Transducer and Method of Making the Same,” issued Aug. 7, 2001, Ladabaum describes improvements to microfabricated acoustic transducers which enable competitive performance with piezoelectric transducers.
The basic transduction element of the conventional capacitive microfabricated ultrasonic transducer is a vibrating capacitor. A substrate contains a lower electrode, a thin diaphragm is suspended over the substrate, and a metallization layer serves as an upper electrode. If a DC bias is applied across the lower and upper electrodes, an acoustic wave impinging on the diaphragm will set it in motion, and the variation of electrode separation caused by such motion results in an electrical signal. Conversely, if an AC signal is applied across the biased electrodes, the AC forcing function will set the diaphragm in motion, and this motion emits an acoustic wave in the medium of interest.
FIGS. 1A–1C illustrate the naming conventions, as well as the conventional focusing and scanning directions, in a typical transducer array used in medical imaging applications. As shown in FIG. 1A, the transducer 100 is typically made up of multiple transducer elements 110. Each of the transducer elements 110 includes a plurality of individual transducer cells. The transducer elements 110 are oriented such that their lengths are along the elevation axis, and their widths are along the azimuth axis. The transducer elements 110 are adjacent to one another along the azimuth axis. As shown in FIG. 1B, a transducer array 100 is conventionally focused to a focal spot 150 in the range direction and scanned in the azimuth direction electronically by applying an appropriate time delay to each of the transducer elements 110. As shown in FIG. 1C, focus in the elevation direction has conventionally been achieved with a mechanical lens 120. This mechanical focus is sub-optimal because adequate elevation focus is only obtained over a relatively small portion of the usable range. Whereas time-delay focusing in the azimuth plane is possible over the entire field of view (for example 0.5–6 cm for a linear small parts probe), elevation focus is only obtained over a relatively small spatial region of peak focus 130. A measure of the elevation focus achievable, as well as the depth of field over which good focus can be achieved, is often described by the term slice thickness.
Traditional ultrasound transducers have poor control of the slice thickness. A convex lens 120 focuses somewhere in the middle of the usable range, and the focus diverges, or becomes large, at ranges beyond the lens' peak focus 130. This divergent lens focusing creates volume averaging artifacts that obscure small cysts and other clinically relevant, yet small feature size, information. Furthermore, practical physical lenses are often lossy, which further decreases their effective use, because they are made from materials whose speed of sound is slower than the speed of sound in a body.
The elevation focus challenge is well known in the art, and various approaches to improve slice thickness, i.e., to decrease slice thickness, have been taught. For example, U.S. Pat. No. 4,670,683 to 't Hoen teaches that electronic phase delays between different sub-elements in the elevation direction can correct for the slice thickness short-comings of a mechanical lens. However, by requiring delay means in both transmit and receive between “central” and “side” electrodes in elevation, in addition to azimuth delay means, this invention necessitates significant system and transducer complexity.
U.S. Pat. No. 5,301,168 entitled, “Ultrasonic transducer system,” filed Jan. 19, 1993, to Miller teaches that a multi-aperture transducer system, where elements are subdivided into multiple elevation apertures, can be used to improve slice thickness. However, the practical complexity of dividing M azimuth channels into N segments in elevation, the N segments grouped into E apertures, is a material disadvantage to this type of design. In order for each of the M channels to be able to have E apertures, switches are necessary in the path of the radio frequency (RF) signal (see, for example, the “combining network” in FIG. 8 of the Miller patent). Furthermore, mechanically segmented mechanical lenses are required to achieve good slice thickness for the varying apertures. Because of the cost and complexity of such an arrangement, it is desirable to have-an ultrasonic probe with good slice thickness that is simpler to make. It is also desirable to have ultrasonic probes with minimal losses due to the lens losses and without the need for combining network switches in the RF path.
U.S. Pat. No. 5,651,365 to Hanafy et al. teaches that slice thickness can be improved by using two sets of interleaved azimuthal transducer elements, each set having a different elevation aperture. One set is used for optimized focus at a certain range, and the second set is used for an optimized scan at a different range. However, this approach negatively impacts at least one of efficiency, lateral resolution, or frame rate.
U.S. Pat. No. 5,415,175 to Hanafy et al. teaches that by varying the thickness and curvature of a piezoelectric element along the elevation direction, that frequency dependent elevation focusing can be achieved. While this invention is known to those skilled in the art as resulting in ultrasound probes with improved slice thickness performance over conventional probes, the elevation aperture is problematic for low-frequency, relatively narrow-band signals such as those emanating from deep within the tissue. Furthermore, fabrication of these curved surfaces is challenging and consequently expensive.
U.S. Pat. No. 6,381,197 to Savord et al. (e.g., FIGS. 5A and 5B of the Savord patent) teaches that bias rows in the elevation direction of a microfabricated ultrasonic transducer (MUT) can be connected to bias sources, and that by using these bias sources to selectively energize elevation rows, the elevation aperture of a MUT can be controlled. The Savord patent further teaches that elevation apodization can be achieved by varying the gain in the elevation direction with the bias rows; inherent in the Savord apodization teaching is the complexity of multiple bias sources each at different voltage amplitudes, which is not desirable in practical applications. As taught by the Savord patent, control of aperture and apodization by varying the magnitude of the bias on a MUT is effective only in receive operation. During transmission, the MUT cannot be effectively turned off by bias amplitude alone and is operated outside of its linear range, that is, with the transmit pulse itself essentially biasing the transducer. Thus, it is desirable to provide a means for aperture control that is equally effective in both transmit and receive operation. It is also desirable to provide a simple means for apodization that is effective in both transmit and receive operation.
It has been realized by the present inventors that a judiciously chosen spatial variation of the sign of the bias voltage is an effective way to control the transmit radiation of a MUT surface. It has been further realized by the present inventors that varying the sign of the bias voltage results in a 180 degree phase shift of the transmitted wavefront, and that this phase shift can be used to create Fresnel lens effects with microfabricated ultrasonic transducers. The Savord patent neither teaches nor suggests such control via spatial distribution of bias polarity.
Thus, the polarity of bias can modulate the phase, in elevation for example, of both the transmitted and received ultrasonic waveform. This bias-polarity-based phase modulation can be used to effectively control the aperture of a MUT device by providing precise cancellation of both transmitted and received acoustic energy. This bias-polarity-based phase modulation can also be used to create focus in the far field without using a mechanical lens, or to enhance focusing when combined with other lensing means. It can also be used to greatly simplify the design and implementation of probes with excellent slice thickness performance.